The declining production of domestic crude oil has placed considerable emphasis on the development of new technology for the extraction of residual oil remaining in the oil field after cost effective primary and secondary methods have been utilized to produce oil from the field. Research developing tertiary methods for oil removal has emphasized the importance of accurately determining the oil saturation distribution in an oil field. Three principal methods currently are used for estimating the fluid saturation as opposed to fluid saturation distributions, in oil field:
(1) Core analysis is the extraction of cores followed by a laboratory analysis of the oil content; PA1 (2) Logging is the interpretation of electrical, acoustical and radioactive signals obtained from specialized tools lowered into representative species of the wells and PA1 (3) Material balance calculations are the estimation of the remaining fluid content in the field by subtracting produced fluid volume from the initial estimates of the reservoir capacities or saturations.
Each of these methods has varying degrees of accuracy depending upon laboratory and sampling techniques. The core analysis method yields the fluid saturation of a small sample which generally is contaminated with fluids used in drilling. The logging technique produces measurements of fluid saturation but only within a few feet of the well bore or the well hole and these measurements may also yield grossly inaccurate estimates of the formation saturation if water coning has occurred at the well. Finally, the material balance calculations are dependent upon the accuracy of the initial oil saturation estimates as well as production records, all of which may be 25 to 40 years old, and also are dependent upon the calculated estimates of pressure decline in the reservoir. Accordingly, none of these methods yield information relative to interwell fluid saturation distribution, unless new wells are drilled throughout the field.
Dry sandstones, unconsolidated sands, carbonate rocks, oil and gas have such high resistance to an electrical current that they are considered to be insulators. Minerals, especially clay and pyrite, frequently associated with the sedimentary rocks that make up most petroleum reservoirs are conductive and therefore complicate the interpretation of resistivity logs. Assuming that the porous rock matrix is non-conductive, then the electrical properties depend only upon the brine or connate water contained in the pores of the rock and the geometry of the pores. Then, the conductivity of a fluid-saturated rock is due to the ions of the dissolved salt that make up the brine and the magnitude of the electrical conductivity depends upon the salt concentration and temperature. Pioneer work in the field was performed by G. E. Archie as set forth in his paper "The Electrical Resistivity Log As An Aid In Determining Some Reservoir Characteristics", Trans. AIME, v. 146, 1942, PP. 54-62.
As is known in the art, the resistivity of a material is the reciprocal of the conductivity and is defined as: EQU .rho.=(r.multidot.A)/L
wherein: .rho. is resistivity expressed in ohm-meters; r is resistance expressed in ohms; A is area expressed in square meters; and L is length of current path expressed in meters. The value of the resistivity of a rock which is completely saturated with brine of a given concentration at a specific temperature was defined by Archie as follows: EQU F=R.sub.o /R.sub.w =.phi..sup.-m
wherein F is the formation resistivity factor; R.sub.o is the resistivity of rock 100 percent saturated with brine expressed in ohm-meters; R.sub.w is resistivity of brine expressed in ohm-meters; .phi. is the porosity and m is an empirical constant. Resistivities of oil field brines have been investigated and values published for varying temperature values. It has been established that the formation factor increases as the sand becomes more cemented, the degree of sand cementation affecting the value of the exponent m.
Archie gives an empirical relationship for water saturation. For clean, water-saturated sands, the average value of n is 2; hence, the water saturation may be estimated with a fair degree of accuracy using the empirical relationship: EQU S.sub.w =(R.sub.o /R.sub.t).sup.1/n =(FR.sub.w /R.sub.t).sup.0.5
wherein: S.sub.w is brine saturation; R.sub.t is resistivity of rock with brine and another fluid; and n is an empirical constant. Accurate information is difficult to obtain when the sedimentary formation being measured contains variable amounts of shale and silty materials. Data have been developed showing that wet rocks containing clay and other minerals was found to be equal to the sum of the conductivity of the rock and the brine. Theories have been advanced that the abnormal electrical characteristics of shaly materials are due to the absorption of ions from the brines by the shaly materials and mathematical interpretations based on laboratory results seem to verify these theories.